Field of the Invention
The present invention relates to a thin film transistor substrate including thin film transistors formed of an oxide semiconductor and a method for manufacturing a thin film transistor substrate.
Description of the Background Art
Thin film transistor (TFT) active matrix substrates (thin film transistor substrates (TFT substrates)) including thin film transistors (TFTs) as switching elements find use in electro-optical apparatuses such as displays (liquid crystal displays) including liquid crystals. The semiconductor devices such as the TFTs have features of being power-thrifty and thin, thus finding increasing applications to flat panel displays as a replacement for cathode ray tubes (CRTs).
The electro-optical elements for use in liquid crystal displays (LCDs) include passive matrix LCDs and TFT LCDs including TFTs as switching elements. In particular, the TFT LCDs are superior in portability and display quality to the CRTs and the passive matrix LCDs, thus finding widespread practical applications to display products such as notebook computers and TVs.
In general, the TFT LCD includes a liquid crystal display panel in which a liquid crystal layer is sandwiched between a TFT substrate and a counter substrate. The TFT substrate includes an array of a plurality of TFTs and the counter substrate includes, for example, a color filter. The liquid crystal display panel includes polarizing plates located on the front surface side and the back surface side. A backlight is located on one of these sides. This structure provides an excellent color display.
The liquid crystal driving systems for liquid crystal displays include the vertical electric field system, such as the twisted nematic (TN) mode and the vertical alignment (VA) mode, and the transverse electric field system, such as the in-plane switching (IPS) (IPS is a registered trademark) and the fringe field switching (FFS) mode. In general, liquid crystal displays employing the transverse electric field system has the advantage over the liquid crystal displays employing the vertical electric field system in the wider viewing angle, the higher resolution, and the higher luminance, thus becoming mainstream for small-to-medium-size panels such as smart phones and tablets.
The liquid crystal display panel employing the vertical electric field system includes a pixel electrode located on the TFT substrate and a common electrode located on the counter substrate. A voltage corresponding to an image signal is applied to the pixel electrode while the common electrode is fixed at a constant electric potential (common potential). Thus, liquid crystals of the liquid crystal layer are driven by the electric field substantially vertical to the surface of the liquid crystal display panel.
Meanwhile, the liquid crystal display panel employing the transverse electric field system includes the pixel electrode and the common electrode that are located on the TFT substrate, and thus, liquid crystals of the liquid crystal layer are driven by the electric field substantially horizontal to the surface of the liquid crystal display panel. In particular, the TFT substrate in the FFS mode includes the pixel electrode and the common electrode disposed opposite to each other with an insulating film located therebetween. Either the pixel electrode or the common electrode may be located below the other. The electrode located on the lower side is formed into a flat plate shape while the electrode located on the upper side is formed into a lattice pattern or a comb-teeth shape having slits.
For the switching element of the TFT substrate to be included in the conventional liquid crystal display, the semiconductor film for forming the active layer (channel layer) of the TFT has been made of amorphous silicon (a-Si). In recent years, the TFTs including an active layer made of an oxide semiconductor are actively developed. Such oxide semiconductor has a mobility higher than that of the conventionally-used amorphous silicon. The oxide semiconductor is mainly the material based on zinc oxide (ZnO) or the material based on amorphous InGaZnO obtained by adding gallium oxide (Ga2O3) and indium oxide (In2O3) to zinc oxide. This technique is disclosed in, for example, Japanese Patent Application Laid-Open No. 2005-77822, Japanese Patent Application Laid-Open No. 2007-281409, and Kenji Nomura, et al., “Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors,” Nature, no. 432 (2004): 488-492.
Similarly to the oxide conductors being the transparent conductors such as amorphous ITO (indium oxide (In2O3)+tin oxide (SnO2)) and amorphous InZnO (indium oxide (In2O3)+zinc oxide (ZnO)), the above-mentioned oxide semiconductor can be etched with a weak acid solution containing oxalic acid or carboxylic acid, thus having the advantage of being easily patterned.
However, in some cases, the oxide semiconductor material is damaged by the acid solution used to etch a general metal film (Cr, Ti, Mo, Ta, Al, Cu, or an alloy containing these metals) which is to be formed into a source electrode and a drain electrode of the TFT, resulting in characteristics degradation. In other cases, certain kinds of oxide semiconductor materials may dissolve in the above-mentioned acid solution. Thus, in a case where the TFT (generally referred to as back channel etching (BCE) TFT) including the source electrode and the drain electrode located on the channel layer made of an oxide semiconductor is formed as shown in FIG. 11B in Japanese Patent Application Laid-Open No. 2007-281409, the channel layer is damaged, in some cases, by the acid solution used to process the source electrode and the drain electrode, resulting in the degradation in the TFT characteristics. In other cases, the channel layer is damaged due to the oxidation-reduction reaction in the interface while the metal film which is to be formed into the source electrode and the drain electrode is deposited on the oxide semiconductor film (channel layer), causing the degradation in the TFT characteristics.
These problems can be solved by, for example, applying the TFT structure that includes a protective insulating layer formed on the semiconductor layer as described in Japanese Patent Application Laid-Open No. 62-235784 (1987). Such TFT structure can prevent damage to or loss of the oxide semiconductor film involved in the etching for processing the metal film to form the source electrode and the drain electrode. The TFTs having this structure are generally referred to as “etching stopper (ES) TFTs” or “etch stopper (ES) TFTs.”
For example, in FIGS. 1 and 2 of Japanese Patent Application Laid-Open No. 2005-77822, the ES TFT substrate in the TN mode is disclosed which includes the channel protective film (channel protective layer) made of silicon oxide or silicon nitride and located on the semiconductor film (channel layer) made of a metal oxide.
In general, the production of the TFT substrate in the TN mode including the back channel etching TFTs in which the a-Si semiconductor film serves as the channel layer as shown in FIGS. 1 and 2 of Japanese Patent Application Laid-Open No. 10-268353 (1998) involves five photolithography processes of: (1) forming a gate electrode; (2) forming a gate insulating film and a channel layer; (3) forming a source electrode and a drain electrode; (4) forming a contact hole in a protective insulating film; and (5) forming a pixel electrode.
The production of the FFS-TFT substrate including the back channel etching TFTs as shown in FIGS. 2 and 3 of Japanese Patent Application Laid-Open No. 2009-151285 involves seven photolithography processes of: (1) forming a gate electrode; (2) forming a gate insulating film and a channel layer, (3) forming a source electrode and a drain electrode; (4) forming a contact hole in a protective insulating film; (5) forming a pixel electrode; (6) forming a contact hole in an interlayer insulating film; and (7) forming a common electrode.
The production of the TFT substrate including the general etch stopper TFTs in which the channel layer is made of an oxide semiconductor requires at least one additional photolithography process for the formation of the protective layer on the oxide semiconductor film. Unfortunately, this additional photolithography process reduces the production capacity and increases the manufacturing cost.
For example, the method for producing the etch stopper TFT substrate in the TN mode is proposed in PCT International Publication No. 2011/077607. This method involves four photolithography processes of: (1) forming a gate electrode; (2) forming a channel layer from an oxide semiconductor; (3) forming a contact hole in a protective insulating film; and (4) forming a pixel electrode, a source electrode, and a drain electrode. (In some cases, another photolithography process for forming a source wire which is to be connected to the source electrode is performed between the process (2) and the process (3).)
In the production of a TFT substrate according to the method disclosed in PCT International Publication No. 2011/077607, a first insulating film being the same layer as the gate insulating film and a second insulating film being the same layer as the protective insulating film reside below the source wire having the source electrode of the TFT connected thereto. The process of etching the oxide semiconductor film is performed between the process of depositing the first insulating film and the process of depositing the second insulating film. In some cases, the surface of the first insulating film is damaged through the process of etching the oxide semiconductor film, resulting in the deterioration in the adhesion between the first insulating film and the second insulating film. Thus, following an extended period of use of the liquid crystal display, the source wire is likely to be broken in the area where the first insulating film and the second insulating film poorly adhere to each other. This could impair the reliability.
PCT International Publication No. 2011/077607 describes the method for reducing the number of photolithography processes for LCDs including the etch stopper TFTs, but makes no mention of the method for reducing the number of photolithography processes and the manufacturing cost for the LCDs employing the transverse electric field system (in particular, FFS LCDs).